1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a dual mode liquid crystal display device convertible between a memory mode and a dynamic mode.
2. Discussion of the Related Art
With the rapid development in information technology, flat panel display (FPD) devices having thin thickness, light weight, and lower power consumption have been introduced and developed.
Among these devices, liquid crystal display (LCD) devices are most widely used for monitors of notebook computers, monitors of personal computers and televisions due to high definition, high qualities, excellent moving images and high contrast ratio.
An LCD device includes two substrates and a liquid crystal layer interposed between the two substrates. Electrodes are formed on respective substrates, and the substrates are disposed such that the electrodes face each other. An electric field is induced between the electrodes when voltages are applied to the electrodes. The alignment direction of the liquid crystal molecules is controlled by varying the intensity of the electric field, and the transmittance of light through the liquid crystal layer is changed to display images.
FIG. 1 is an exploded perspective view of an LCD device according to the related art. As shown in FIG. 1, the LCD device includes an array substrate 10, a color filter substrate 20 and a liquid crystal layer 30. The array substrate 10 and the color filter substrate 20 face each other, and the liquid crystal layer 30 is interposed therebetween.
The array substrate 10 includes gate lines 14 and data lines 16 on an inner surface of a transparent substrate 12. The gate lines 14 and the data lines 16 cross each other such that regions formed between the gate and data lines 14 and 16 are defined as pixel regions P. A thin film transistor Tr is formed at each crossing portion of the gate and data lines 14 and 16, and a pixel electrode 18 is formed in each pixel region P and connected to the thin film transistor Tr.
The color filter substrate 20 includes a black matrix 25, a color filter layer 26, and a common electrode 28 on an inner surface of a transparent substrate 22 facing the array substrate 10. The black matrix 25 has a lattice shape to cover a non-display region such as the gate lines 14, the data lines 16, the thin film transistors Tr, and so on. The color filter layer 26 includes red, green and blue color filter patterns 26a, 26b, and 26c repeatedly arranged in order. Each of the color filter patterns 26a, 26b, and 26c corresponds to each pixel region P. The common electrode 28 is formed on the black matrix 25 and the color filter layer 26 and over an entire surface of the substrate 22.
A sealant (not shown) is formed along peripheries of the array substrate 10 and the color filter substrate 20 to prevent liquid crystal molecules of the liquid crystal layer 30 from leaking An alignment layer (not shown) is formed between the liquid crystal layer 30 and each of the array substrate 10 and the color filter substrate 20 to determine an initial direction of the liquid crystal molecules. First and second polarizers (not shown) are disposed on outer surfaces of the array substrate 10 and the color filter substrate 20, respectively. A backlight unit (not shown) is disposed over an outer surface of the array substrate 10 to provide light.
Scan signals for turning on/off the thin film transistors Tr are sequentially applied to the gate lines 14, and data signals are applied to the pixel electrodes 18 in the selected pixel regions P through the data lines 16. An electric field perpendicular to the substrates 12 and 22 is induced between the pixel electrodes 18 and the common electrode 28. The arrangement of the liquid crystal molecules is controlled by the electric field, and the transmittance of light is changed by varying the arrangement of the liquid crystal molecules to thereby display various images.
In the LCD device, the liquid crystal molecules may exhibit nematic, smectic or cholesteric phases. Among these, the nematic phase has been most widely used because light is strongly scattered when the liquid crystal molecules are dispersed.
The electro optic effect of the liquid crystal means a phenomenon of generating optical modulation by changing optical characteristics of a liquid crystal cell and is caused by a change in arrangements of the liquid crystal molecules due to an electric field.
In the nematic phase, the arrangements of the liquid crystal molecules are continuously changed when an electric field is applied. A twisted nematic (TN) mode and a super twisted nematic type (STN) mode are widely used for an LCD device including a nematic phase liquid crystal.
A TN mode LCD device includes a liquid crystal panel where nematic phase liquid crystal molecules are disposed between transparent electrodes which are surface-treated to have an angle of 90 degrees with respect to each other. The liquid crystal molecules are parallel to the electrodes and are continuously twisted by 90 degrees from one electrode to another.
Meanwhile, to increase viewing angles, an in-plane switching mode LCD device has been suggested in which a common electrode and a pixel electrode are formed on the same substrate. The liquid crystal molecules are rotated by a horizontal electric field parallel to the substrate.
Recently, various types of LCD devices have been developed to satisfy needs of consumers. Specially, LCD devices having further thin thickness, light weight and high efficiency have been proposed to watch moving images or to read texts when they move.
Therefore, it is needed to provide an LCD device including a dynamic mode for watching moving images and a memory mode for reading texts.
An LCD device including a dynamic mode and a memory mode according to the related art uses bistable states of a splay state and −π twist state for the memory mode and switching between a low bend state and a high bend state for the dynamic mode. This LCD device may be referred to as a bistable chiral splay nematic (BCSN) mode LCD device.
The BCSN mode LCD device includes different black states for the memory mode and the dynamic mode. Thus, it is difficult to design a compensation film for satisfying both the memory mode and the dynamic mode.
Namely, a black property is optimized for only one of the memory mode and the dynamic mode. The black property for the other mode is lowered, and the contrast ratio is also lowered. The LCD device of high qualities is not provided.